The present invention relates generally to electrolytic capacitors. In particular, the present invention relates to maintaining wet-tantalum capacitors used in medical devices to deliver high energy electrical therapy to a patient.
Implantable cardioverter defibrillators (ICD's) and automatic external defibrillators (AED's) apply a therapeutic electric shock to a patient's heart to restore the heart to a normal rhythm. These devices use high voltage capacitors that are charged just before the cardioversion or defibrillation therapy is delivered, and then discharged through electrodes to deliver the therapeutic electrical shock. Wet electrolytic capacitors are typically used in ICD's and AED's. A wet electrolytic capacitor includes a metallic anode, a metal oxide layer formed on the anode, a liquid electrolyte, and a cathode.
Originally, aluminum electrolytic capacitors having an aluminum anode with an aluminum oxide coating were used. More recently, wet-tantalum capacitors having a tantalum anode, a tantalum oxide dielectric layer, a liquid electrolyte, and a cathode (e.g., a tantalum or ruthenium oxide) have been developed for use in ICDs and AEDs.
When wet electrolytic capacitors rest on open circuit for days or longer, a process commonly referred to as “deformation” occurs. As a result, when the capacitor is next charged, an appreciable amount of energy is used to “reform” the oxide dielectric layer. This results in longer than desired charging times for the ICD or AED. It also affects the longevity of the device, because a greater amount of energy from the battery is required during the charging process.
Techniques for reforming electrolytic capacitors in ICD's are discussed in Kroll U.S. Pat. No. 5,741,307; Startweather et al. U.S. Pat. No. 5,792,188; Kroll U.S. Pat. No. 5,861,006; and Silvian U.S. Pat. No. 6,096,602. These patents describe reform techniques which were originally used with aluminum electrolytic capacitors.
Wet-tantalum capacitors exhibit less severe deformation than aluminum electrolytic capacitors, but degradation of wet-tantalum capacitors and techniques for reforming the tantalum/tantalum oxide anode have also been addressed. Methods of reforming wet-tantalum capacitors are described in Harguth et al. U.S. Pat. Nos. 6,283,985 and 6,706,059, Liu et al. Publication No. U.S. 2003/0088273, now abandoned; and pending Norton et al. Publication No. U.S. 2004/0225327.